This application claims the benefit of Korean Patent Application Serial No. 2001-1654, filed Jan. 11, 2001.
1. Field of the Invention
The present invention relates in general to a measurement device for measuring the displacement of a target (object to be measured) at a high degree of positioning precision, for example, several ten nanometers, and more particularly to a high-precision displacement measurement device using a unit displacement sensor based on a confocal theory, which is simpler in construction, lower in cost and superior in resolution to other displacement measurement devices, and is capable of measuring a much wider area than other measurement equipment.
Further, the present invention relates to a displacement measurement method for measuring a linear or rotational displacement using a unit displacement sensor based on a confocal theory.
Therefore, this invention can be used as a measurement sensor for a high-precision positioning mechanism such as equipment for feeding and aligning wafers in a semiconductor lithographic process. Other than this, the present invention can be used as a sensor for high-precision positioning control of machine tools, feeding devices for optical precision equipment, high-density storage media, etc.
Further, the present invention relates to a device capable of replacing optical interferometers or capacitive displacement measurement devices used for high-precision displacement measurements.
2. Description of the Prior Art
A capacitive displacement measurement device is one of conventional displacement measurement devices. This displacement measurement device is a high-precision displacement sensor using variations in the flow of charges with variations in inter-plate interval. This device is so very precise that the amount of noise in a circuitry can be directly connected with a positioning resolution, but it is disadvantageous in that it can perform a measuring operation only within the range of a short distance and is high in cost due to a high sensitivity to vibrations.
Another conventional displacement measurement device is an encoder using an optical disk, which is a sensor for measuring a rotational displacement of a target. This encoder is adapted to obtain the amount of rotation on the basis of the number of beams of light transmitted through holes arranged at regular intervals in a circumferential direction of the optical disk, as in most encoders. This encoder is limited in rotational displacement measurement resolution because this resolution is determined according to the number of holes provided in the optical disk.
Another conventional displacement measurement device is an optical grating sensor, which is a displacement sensor using a diffraction of a beam of light through a grating depending on optical conditions such as a beam wavelength, grating interval, angle of incidence, etc. The optical grating sensor is adapted to obtain a linear displacement of the grating using variations in angle of the diffracted light beam with variations in the grating interval among such optical conditions. However, the optical grating sensor is disadvantageous in that gratings having different intervals must be precisely fabricated, and the diffracted beam of light has a high sensitivity to the diffraction conditions and a great nonlinearity in light amount. In other words, the measurement of a relative displacement between the optical system and the grating requires a very accurate setup and is not excellent in position detection resolution.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a high-precision displacement measurement device using a unit displacement sensor based on a confocal theory, which is simpler in construction, lower in cost and superior in resolution to other displacement measurement devices, and is capable of measuring a much wider area than other measurement equipment.
It is another object of the present invention to provide a displacement measurement method for measuring a linear or rotational displacement using a unit displacement sensor based on a confocal theory.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a high-precision displacement measurement device for projecting a spot of light from a light source on an object whose displacement is to be measured and measuring a relative displacement of the object from a displacement of the projected light spot, the device comprising a unit displacement sensor based on a confocal theory, the unit displacement sensor including a transmission lens for transmitting a divergent beam of light emitted from the light source through a slit or pinhole with a certain size, a collimating lens for collimating the light beam transmitted through the slit or pinhole, an object lens arranged between the collimating lens and the object, a beam splitter for splitting a beam of light, reflected from the object to the transmission lens, according to a wavelength, and a photodetector for detecting a light intensity of the light beam split by the beam splitter; and actuation means for finely moving the unit displacement sensor to always maintain optical signals detected by the photodetector at the maximum intensity.
Preferably, the actuation means may be a piezoelectric actuator attached to the unit displacement sensor.
Further, an integrated unit of a laser diode and photodiode may replace the light source, beam splitter and photodetector.
In accordance with another aspect of the present invention, there is provided a short-distance displacement measurement method using a unit displacement sensor based on a confocal theory, which projects a spot of light from a light source on an object whose displacement is to be measured and measures a relative displacement of the object from a displacement of the projected light spot, the method comprising the steps of a) by a transmission lens, transmitting a divergent beam of light emitted from the light source through a slit or pinhole with a certain size; b) by a collimating lens, collimating the light beam transmitted through the slit or pinhole; c) by an object lens, projecting the light beam collimated by the collimating lens on the object; d) by a beam splitter, splitting a beam of light, reflected from the object to the transmission lens, according to a wavelength; e) by a photodetector, detecting a light intensity of the light beam split by the beam splitter; and f) detecting a displacement of actuation means minutely moving relatively while tracing a fine movement of the object, thereby always maintaining optical signals detected by the photodetector at the maximum intensity.
In accordance with a further aspect of the present invention, there is provided a long-distance linear displacement measurement method using a unit displacement sensor based on a confocal theory, comprising the step of attaching a linear grating of a known shape to a long-distance object whose rotational displacement is to be measured, linearly moving the unit displacement sensor in close proximity to the linear grating and measuring a linear displacement of the long-distance object from successive optical signals of the same response frequency detected by the unit displacement sensor.
In accordance with yet another aspect of the present invention, there is provided a rotational displacement measurement method using a unit displacement sensor based on a confocal theory, comprising the step of attaching a circumferential grating of a known shape to an object whose rotational displacement is to be measured, in a circumferential direction of the object or forming the grating directly in the object in the circumferential direction of the object, moving the unit displacement sensor in close proximity to the circumferential grating and measuring a rotational displacement of the object from successive optical signals of the same response frequency detected by the unit displacement sensor.